The correct option is D Investigation is mandatory in nature and needs to be done on yearly basis.
Investigation - Investigation is an examination of books and records for any specified purpose, sometimes differing in scope from the ordinary audit. Investigation implies an examination of and record for some special purpose. Investigation is carried out not in substitution of audit, but in addition of audit. Investigation is carried out on behalf of the outsiders who want to know the financial position of the business. Investigation can be conducted by anyone and he may not be an auditor. There is no format of investigation report.